1. Field of the Invention
This invention relates to compositions and processes for desmutting metal surfaces, particularly the surfaces of aluminum and magnesium and their alloys that contain at least 45 % by weight of aluminum or magnesium, more particularly aluminum alloys containing sufficient copper to form smut on their surfaces easily when dissolving. The compositions and processes are also useful for deoxidizing unsmutted surfaces and achieve deoxidizing without forming smut on the surfaces or otherwise staining them. ("Deoxidizing" is to be understood herein as the removal from the surface of metals of oxide films and other adherent inorganic materials that would reduce adhesion to subsequently desired protective coatings such as conversion coatings and/or paints and the like. With most deoxidizing agents, there is a perceptible but controlled dissolution of the underlying metal while the deoxidizing agent is in contact with it. In contrast, "desmutting" is to be understood herein as the removal, without significant attack on the underlying metal, of powdery and usually darkly colored, residues produced on a treated metal surface by some prior cleaning, etching, and/or deoxidizing treatment.)
2. Statement of Related Art
Since the development of copper containing aluminum aerospace alloys several decades ago, the conventional deoxidizing compositions have normally included concentrated nitric and/or sulfuric acid and chromates, with fluorides, ferric ions, oxidizers such as persulfate and peroxide, and ferricyanide all serving as frequently used optional ingredients. In recent years there has been environmentally driven incentive to avoid chromates and ferricyanides, but no fully satisfactory deoxidizer free from these materials is believed to have been developed.